School buses include safety units mounted in different locations for performing two main functions: first, some school buses include an indicator which may extend out from the side of the bus to alert traffic to stop when the bus is stopped; second, some buses also include an elongated gate mounted on the front of the bus for directing children around the front of the bus and in a predetermined path visible to the driver, preventing children from lingering directly in front of the bus where the driver cannot see. The same mechanism may be used for mounting and controlling the movement of both safety units, i.e. the indicator and the elongated gate. The mechanism typically includes a motor and a bracket interconnected by some type of clutch mechanism, with an electronic control circuit for controlling the motor to move the bracket, and thus the attached safety unit, between a retracted position and an extended position. The clutch mechanism allows movement of the bracket with respect to the motor when a force acts on the bracket. Such a force may result when the safety unit is left extended, and the safety unit hits a tree or another vehicle.
Several types of clutch mechanisms are shown in the prior art. U.S. Pat. No. 4,983,949 to Wicker, for example, teaches a slip clutch which disengages the pivot arm from the drive mechanism when a predetermined force acts on the pivot arm; however, this mechanism is unnecessarily complex and expensive to manufacture. U.S. Pat. No. 4,766,413 to Reavell teaches a drive mechanism interconnected with a safety gate by means of a lever arm and a compression spring for allowing relative rotation between the safety unit and the drive mechanism; however, this mechanism is also unnecessarily complex and expensive to manufacture.